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The high-pressure, high-temperature behavior of transition metal dioxides TiO2, ZrO2, and HfO2 has been reviewed. In particular, early predictions and measurements that suggested superhard behavior have been considered. In subsequent studies, it has been concluded that superhardness is not intrinsic in the transition metal dioxides at ambient conditions or to phases produced at high-pressure and/or high-temperature conditions.
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Al-Khatatbeh et al. (2014) studied this question.
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